An optical isolator may be used in an electrical circuit to allow signal propagation in a forward direction while maintaining voltage and current isolation between the input and the output of the optical isolator. Such an isolator includes a light emitting diode (LED) at the input, a photodiode at the output and an optically transmissive high breakdown voltage isolation gap separating the LED and the photodiode. In order to achieve optimum isolator performance the size of the isolation gap and the alignment of the LED and the photodiode must be maintained accurately during manufacture.
One prior art attempt to automate optical isolator manufacture, described in U.S. Pat. No. 4,446,375 and shown in FIGS. 1A-C hereof, has been to align the LED and the photodiode during fabrication by folding the LED lead and the separate LED bond lead into place over the detector in two ninety degree steps. Although the use of such a folded lead frame technique decreases the assembly time for each isolator a complex folding apparatus is required to make the two 90.degree. folds which lie along different axes and errors in the placement of the folds may affect the accuracy of the LED-photodiode alignment. During folding, damage to the bond wires may occur because the LED lead and the LED bond lead are not securely attached together and because the dielectric sheet between the LED and the photodiode dice is free to rest against and damage the LED and photodiode bond wires. As additionally described in U.S. Pat. No. 3,925,801 and shown in FIG. 2 hereof, the size of the isolation gap in many prior art optical isolators is determined by the thickness of a dielectric sheet located between the LED and the photodiode. The insertion of such a sheet during manufacture has often resulted in bond wire damage.
In accordance with the illustrated preferred embodiment of the present invention, an optical isolator is manufactured simply and repeatably using standard form lead frames. Prior to die attachment and wire bonding of the optical isolator, the LED and photodiode attachment portions of the lead frame are displaced to a common plane located below the plane of the remainder of the lead frame. The entire LED section of the lead frame is then rotated 180 degrees about a pair of hinge pins until it is coplanar with the photodiode section of the lead frame. After rotation is completed the hinge pins are stamped to eliminate spring back of the rotated LED section of the lead frame. A clear resin is injected between two dielectric sheets attached to the outer surfaces of the LED and photodiode leads to form a light guide between the LED and the photodiode. The optical isolator is encapsulated and the protruding leads are cut and bent to the desired final package shape. In an alternate preferred embodiment of the present invention, a first dielectric sheet is attached to the outer surface of either the LED or the photodiode lead and a second dielectric sheet is positioned between the LED and the photodiode dice. The second sheet rests upon (and adheres to adhesive on) the first sheet and extends between the dice at an angle so that damage to the dice and to the bond wires is avoided.